如何确保所有wordpress和woocommerce功能都包含在自定义代码中？

Question

I'm trying to run a custompiece of code but I just have this in it's own PHP file. How am I supposed to make sure that it 'inherits' all the functions of wordpress and woocommerce so that it knows what to do?

$order = wc_get_order( $order_id );
$order->update_status( 'completed' );